The invention relates to a method for monitoring the power output of the individual cylinders of a multi-cylinder internal combustion engine, for example in order to be able to identify combustion misfires in one or more cylinders in mixture compression internal combustion engines, and cylinders with different energy output in diesel combustion engines, resulting in poor idle quality and poor quiet running.
Combustion misfires in mixture compression internal combustion engines with lambda control and exhaust gas catalytic converter can result in significant thermal damage of the catalytic converter, if subsequent reactions of the non-burned fuel-air mixture occur in the exhaust gas tract as a result. High catalytic converter temperatures also represent a fire hazard for the vehicle. In addition, the exhaust gas quality is significantly worsened by combustion misfires, independent of whether the cause for the combustion misfires is found in errors of fuel measurement or of firing.
Methods for the detection of combustion misfires are already known, which are based on an analysis of the progression of the instantaneous speed of the internal combustion engine, i.e. the angular velocity of the crank shaft during a cycle. Combustion misfires result in special speed progression changes as compared with the normal state. In this method, the instantaneous speed is determined from the time during which the crank shaft rotates around a defined crank shaft angle range. The determination of these crank shaft ranges is carried out using markings on a wheel driven by the crank shaft. Usually, these markings are formed on the flywheel by the teeth of the starter gear crown, but a separate disk driven by the crank shaft can also be provided with markings. The markings generate a corresponding signal in an inductive transducer. The time measurement takes place by counting the pulses of a pulse transmitter which fall within the crank shaft angle range. The determination of the phase position which is necessary for assignment of the individual cylinders is carried out by means of a phase signal, which is triggered once per cycle (in the case of four-stroke internal combustion engines, with two revolutions of the crank shaft per cycle, e.g. by the cam shaft).
The known methods use a progression of the instantaneous speed which is highly resolved via the crank shaft angle. There, the time duration of very narrow crank shaft angle ranges is determined, which makes high cycle rates necessary for the pulse transmitter, in view of the required accuracy. In the method described in U.S. Pat. No. 4,532,592, crank shaft angle ranges of 6.degree. are scanned with a pulse rate of 5 MHz. However, disruptive influences due to cyclical variations in the combustion progression as well as due to production tolerances of the gear crown represent significant problems in such high-resolution instantaneous speed values.
The same problems occur in the method pursuant to DE-A 28 38 927, in which a cylinder-selective comparison of the reference/actual output is carried out on the basis of the high-resolution instantaneous angular velocity, i.e. the angular acceleration.
In DE-A 37 24 420, a method is described, in which instantaneous speed values within a motor cycle are determined at least twice as often as corresponds to the number of cylinders, and combustion misfires are recognized by a comparison of minimum and maximum values assigned to each cylinder. This method requires a great amount of calculation effort.
In a similar method pursuant to DE-A 36 15 547, differences are formed for each cylinder from maximum and minimum instantaneous speed values assigned to each cylinder. To recognize combustion misfires, the difference value of a cylinder is placed in a ratio with the difference value of the previous cylinder in the firing sequence in each case, and a check is carried out to see whether this ratio is below a threshold. If, however, combustion misfires occur in two consecutive cylinders, the misfire in the second cylinder cannot be determined with this method.
A major disadvantage of the known methods consist of the fact that the influence of dynamic engine operation (acceleration, braking) is not taken in consideration in the recognition of combustion misfires.